Clinical Trial Eligibility: How Biomarkers Shape Cancer Treatment Criteria

When you hear about a new cancer drug in the news, it’s easy to assume it’s available to anyone with that type of cancer. But the reality is far more precise. Today, biomarkers are the gatekeepers of clinical trial access - and increasingly, of real-world treatment options. If you or someone you know is considering a clinical trial for cancer, understanding how biomarkers and inclusion criteria work isn’t just helpful - it’s essential.

What Are Biomarkers, Really?

Biomarkers aren’t science fiction. They’re measurable signs in your body that tell doctors something important about your disease. These can be genes, proteins, hormones, or even fragments of DNA floating in your blood. In cancer, they help answer critical questions: Is this tumor driven by a specific mutation? Will this drug work for you? How aggressive is it likely to be?

The FDA defines them as objective measurements that reflect normal biology, disease processes, or how your body responds to treatment. There are seven types, but in cancer trials, three matter most:

• Predictive biomarkers - tell you if a drug is likely to work (like HER2 in breast cancer or EGFR in lung cancer).
• Prognostic biomarkers - indicate how the disease might progress, regardless of treatment (like high levels of PSA in prostate cancer).
• Pharmacodynamic biomarkers - show whether the drug is hitting its target inside your body.

These aren’t theoretical. In 2022, 92% of all new cancer drug approvals by the FDA came with a biomarker-defined patient group. That means if you don’t have the right biomarker, you won’t be eligible - even if your cancer looks the same on a scan.

Why Biomarkers Changed Everything

Before biomarkers, cancer trials were like throwing a wide net. Doctors enrolled patients based on tumor location and stage - say, “Stage 3 lung cancer.” But tumors with the same appearance can behave completely differently. One patient might respond for years. Another might see no benefit at all. That’s why early trials failed so often.

Now, trials use biomarkers to filter patients before they even start. A 2021 analysis of over 9,700 cancer drug programs showed that trials using biomarkers had nearly double the chance of regulatory approval. Phase 2 success rates jumped from 27% to almost 50%. That’s not a small improvement - it’s a revolution.

Take HER2-positive breast cancer. Before targeted therapies, response rates to chemo hovered around 12%. When trials started selecting only patients with HER2 overexpression, response rates tripled to 32%. That’s the power of matching the right drug to the right person.

What’s in the Inclusion Criteria?

Inclusion criteria are the rules that decide who can join a trial. For biomarker-driven trials, they’re not just about age or prior treatments anymore. They’re molecular.

Here’s what you might see in a real-world trial protocol:

• “Must have metastatic non-small cell lung cancer with an EGFR exon 19 deletion or L858R mutation confirmed by NGS testing.”
• “Must have received no more than two prior lines of systemic therapy.”
• “Must have adequate liver and kidney function (lab values within specified ranges).”
• “Must have a tumor sample available for biomarker testing, or agree to a new biopsy.”

Some trials require a tissue biopsy. Others accept a blood test - called a liquid biopsy - which looks for tumor DNA in your bloodstream. Liquid biopsies are faster, less invasive, and becoming more common. In 2023, 31% of Phase 2+ cancer trials used them, up from just 9% in 2020.

But here’s the catch: Not every hospital can run these tests. You need a lab that’s CLIA-certified and experienced in molecular diagnostics. If your local oncologist doesn’t have the capability, you might need to travel to a major cancer center.

The Hidden Hurdles

Biomarkers sound perfect - and they are, in theory. But in practice, they bring new problems.

One big issue? Geographic inequality. A biomarker like HLA-A*02:01 - used in some cell therapies - shows up in over 50% of people in parts of Europe, but less than 20% in some North American populations. That means a trial might be easy to enroll in London but nearly impossible in rural Ohio.

Then there’s timing. Biomarker testing can take 7 to 14 days. For someone with aggressive cancer, that delay can mean losing a window of opportunity. One survey of clinical sites found that 68% of coordinators cited testing delays as their top enrollment barrier.

And even when you’re eligible, you might not stay eligible. Tumors evolve. A biomarker that was positive at diagnosis might be gone after treatment. Some newer trials now allow “dynamic eligibility” - retesting during the trial to adjust who stays in.

Who’s Getting Left Behind?

The biggest concern isn’t science - it’s access. Biomarker testing requires infrastructure: labs, trained staff, funding, and time. Small hospitals and community clinics often can’t afford it.

A 2022 survey found that 82% of sponsors reported inconsistent testing protocols across trial sites. That means two patients with the same cancer might get different tests - and one might be excluded because the test wasn’t done right.

Patients without insurance, those in rural areas, or those from underrepresented racial groups are more likely to face these barriers. The FDA and EMA are pushing for better equity, but progress is slow. As of 2023, 68% of biomarkers used in early trials still lack full analytical validation - meaning the tests themselves aren’t reliable enough for regulatory decisions.

What’s Next?

The future is multi-omic. Instead of testing one gene, trials are starting to look at dozens - combining DNA, RNA, protein, and immune markers into a single profile. By 2025, 65% of new trials are expected to use these panels.

Artificial intelligence is helping too. Top pharmaceutical companies now use AI to find new biomarkers from massive datasets of patient records and genomic data. Some trials are even using real-world data - information from electronic health records - to validate biomarkers faster.

Decentralized trials are growing. Patients can now send blood samples by mail to central labs. Some are even using home collection kits for tissue biopsies. These changes could make trials more accessible - if they’re done right.

What Should You Do?

If you’re exploring a cancer clinical trial:

1. Ask your oncologist: “Is there a biomarker test I should have before considering a trial?”
2. Find out if your hospital can run the test - or if you’ll need to go elsewhere.
3. Ask how long testing takes. If it’s longer than two weeks, ask if a liquid biopsy is an option.
4. Request a copy of your biomarker results. Keep them in your personal medical file.
5. Don’t assume you’re ineligible. New trials open every week. A biomarker that wasn’t testable last year might be standard today.

Remember: Biomarker eligibility isn’t a rejection. It’s a matchmaker. It’s not about being “too sick” or “not sick enough.” It’s about matching your biology to the best possible treatment.

There’s no magic bullet - but there’s a growing number of precise ones. And if you’re eligible, you’re not just participating in a trial. You’re helping shape the next standard of care.